During dynamic braking, traction motors may function as generators to slow the movement of the locomotive by converting the kinetic energy of the locomotive into electrical energy. In rheostatic dynamic braking, grid resistors can be incorporated to dissipate the generated energy as heat. As the locomotive slows, the current generation of the armature decreases. To allow dynamic braking to continue as current generation of the armature continues to decrease, the field current is increased towards the maximum rated current of the field windings. Once the maximum rating of the field current is reached, the equivalent resistance of the grid resistor must be decreased for dynamic braking to continue to function. Lowering the equivalent resistance of the grid resistors permits dynamic braking despite lower rotational velocity of the motors. Early dynamic braking systems begin shorting out portions of the grid resistor to lower its equivalent resistance. This would extend the range of locomotive speed over which dynamic braking was operable. The early dynamic braking systems present two problems.
First, these dynamic braking systems could not operate at slow locomotive speeds. Early braking systems could not shunt out the entire grid resistor without disconnecting the grid blower, a necessary component for dynamic braking, which is in the same circuit as the grid resistor. Typically, once the locomotive has reached speeds of 6 mph or slower, the locomotive must rely solely on its mechanical braking components, like pneumatic brakes, until the locomotive has completely stopped. The increased use of friction-based braking systems produces unnecessary wear on these parts and requires more frequent repairs of the braking system. Thus, to increase the working life of the locomotive's mechanical braking components, an extended range dynamic braking system may be required.
Second, early rheostatic braking systems are only capable of extended range dynamic braking, and cannot produce a continuously linear braking force. Extended-range dynamic braking shorts out portions of the grid resistor at a series of discrete points, such that lowering the equivalent resistance of the grid resistor is accomplished in a series of steps. In a legacy system, each step down causes locomotive handling problems for the operator. During a step-down of the grid resistor, the locomotive often lurches or lunges. The operator must be able to handle each of these difficulties to safely control the locomotive. A braking system capable of applying a continuously linear braking force is able to gradually decrease the equivalent resistance of the grid resistor, eliminating the locomotive handling problems associated with step-down dynamic braking. Thus, to increase locomotive safety, a continuously variable dynamic braking system may be required.
One solution for maintaining dynamic braking at low speeds is described in U.S. Patent Application Publication No. 2009/0295315 A1 (“the '315 publication”). The '315 publication is directed to a system that purportedly incorporates a dynamic braking system for a hybrid locomotive that works at low speeds.
The dynamic braking solution provided by the '315 publication is limited to traction motors in which the armature and the field winding of the traction motor are connected in series. This circuit requires additional control methods to overcome the instability resulting from the armature and field windings being connected in series. For example, these instabilities can create a positive feedback condition across the field coils, which will cause a runaway current build-up if not properly controlled. To implement rheostatic braking, the circuit of the '315 publication requires the current path through the DC bus to be completed by a grid resistor. In this system, connecting the grid resistor directly to the DC bus requires additional controls to prevent the grid resistor from unnecessarily dissipating power during normal motoring. Furthermore, the system cannot isolate the grid resistor in the event it malfunctions. While the '315 publication purportedly allows for dynamic braking at low speeds, the design makes the traction motor drive system unnecessarily vulnerable to current instability and grid resistor breakage.
The presently disclosed traction motor drive system is directed to overcoming one or more of the problems set forth above and/or other problems in the art.